Process for treating metals



Fatented Apr. 17, 1934 UNITED STTS error PROCESS FOR TREATING METALS of Delaware No Drawing. Application December 24, 1929, Serial No. 416,296

Claims.

This invention relates to the treatment of ductile non-ferrous metals such as copper to produce a product of a predetermined temper, or intermediate hardness.

5 The invention relates specifically to a process which consists in treating the metal to modify its structure by subjecting it to a high degree of cold working and then subjecting it to an annealing only sufficient to obtain the desired temper and no more.

The product not only possesses all of the desirable characteristics of completely annealed or softened, and subsequently re-rolled metal, but is formed substantially without scaling and with- 5 out destroying surface texture and color. The cost of producing the sheet is lessened because long continued soaking in the annealing furnace at relatively high temperatures is eliminated and also the necessity for the subsequent rerolling of the sheet to bring it to the desired temper, or

hardness, is avoided.

There are also numerous incidental advantages in this process which affect the quality of the product and the speed of production.

The invention further consists in the new and novel features of operation and the new and original arrangements and combinations of steps in the process hereinafter described and more particularly set forth in the claims.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended here'- to, the invention itself, as to its objects and advantages, the mode of its operation and the man- 35 ner of its organization may be better understood by referring to the following description in which a particular commercial embodiment thereof is disclosed. It will be understood, however, that the processes and the steps thereof may be modified in various respects without departing from the broad spirit and scope of the invention.

In the following description and in the claims the various steps in the process and the details comprising the invention will be identified by specific names for convenience but they are intended to be as generic in their application as the art will permit.

The invention will be described as applied to copper products by way of illustration only. It is to be understood that it is applicable to other (non-ferrous) metals and certain alloys such as nickel silver alloys and the hardness range may be varied to meet the particular requirements.

The process essentially consists in subjecting the copper, or other metal, to a predetermined large amount of cold working so that on subsequent annealing the hardness of the metal remains relatively high at that critical temperature where the annealing curve, which represents the 0 hardness as a function of the temperature, decreases in slope. Below this temperature, which may be called the semi-annealing point, the hardness changes rapidly for a small change in temperature, whereas above this point the hardness 5 changes more gradually as the temperature is increased, and it is possible to control the operation in a commercial process, so as to produce consistently a desired hardness.

It is to be understood that the length of time 7 for which the metal is held at one temperature above this semi-annealing point, has a comparatively slight effect upon the hardness, provided the time is sufficient to raise the various parts of the metal to that temperature. For example, the 7 metal could be held at a given temperature from hour to 12 hours with only a slight difference in resulting hardness.

It has further been discovered that the hardness at this semi-annealing point and above can be controlled, within certain limits, by the percentage of cold reduction to which the metal had been subjected previous to the annealing operation.

In carrying out our process, therefore, we cold work the metal to set up a predetermined high degree of strain hardness. The sheet or other product is then annealed by raising it to a temperature carefully controlled at a point at or above the critical temperature or semi-annealing point. The annealing is continued until the desired temper is obtained.

For example, in annealing copper that has been subjected to very severe rolling, forging, drawing or other mechanical cold reduction, in excess of 75% of its cross sectional area, the softening starts at a temperature between 150 and 350 0., depending on the oxygen, silver and arsenical content of the copper, the amount of cold reduction, and the time of exposure to the temperature, and proceeds very quickly with increase in temperature to a point only about halfway toward dead soft copper. This occurs at a temperature of 50-150 0. above the temperature at which softening starts and may be called the shoulder point of the softening curve.

From here, as the temperature is further increased, further softening for each increment of temperature is markedly less even for long continued exposure, and continues to diminish proquent annealing characteristics.

gressively as the temperature rises so that copper excessively cold worked must often be raised to 700 or 800 C. before it can be considered completely soft.

It is obvious that it would be extremely diificult to stop the annealing operation at any predetermined degree of hardness lying in the range of the initial quick anneal, inasmuch as there the softening effect changes very rapidly with both time and temperature and a large variation in hardness in the final product would be obtained. However, above the semi-annealing point, the time of annealing may vary within wide limits without producing a marked change in hardness of the final product, and the change in resulting hardness with change in temperature is moderate. It is desirable, therefore, to so control the material that the annealing operation can be terminated at or above the semi-annealing point, and still obtain the finished copper product having the hardness required for its subsequent use.

The present process is particularly described as applied to the production of a copper sheet having a hardness on the Rockwell-B scale ball with 100 kgload) load between 68 and '74, which is the hardness usually required for cornice metal. The process may be controlled, however, to produce various other degrees of hardness which may be required for other purposes. For example, a hardness of 67 may be required for light cupping operations, 62 for medium spinning, and heavy cupping, and -40 for severe spinning and and multiple drawing operations.

In carrying out the present process on sheets, a copper slab is first reduced in thickness in the slabbing mill in any manner known in the art and is then applied to a cold rolling mill wherein the thickness is further reduced to produce a sheet having the required dimensions.

When the slab is applied to the slabbing mill, it is usually hot and automatically anneals itself between successive passes. As the operation is continued, however, the material becomes cool and this automatic annealing stops. From this point the material may be considered as cold rolled and the total percentage reduction from the point at which the reannealing stops determines the hardness of the sheet and its subset will be obvious, however, that the process may be applied to copper having a predetermined hardness which has been obtained in any convenient manner. For example, the copper may be completely annealed after hot working and prior to the subsequent cold reduction, or it may be treated in various other ways, as will be apparent to a person skilled in the art.

For producing copper sheets of a temper suitable for forming cornices and for other like uses,

' the cold reduction of between and 92 percent in area is used. The sheet after being cold rolled to effect this reduction has a hardness on the Rockwell-B scale /8" ball) of 100-102. It is then heated to and maintained at the proper temperature a sufficient period to bring the temperature of the sheet to or slightly above the semi-anneal ing point. This point varies in accordance with the amount of previous cold reduction and the chemical composition of the copper. In general, for the same quality of copper the hardness of the semi-annealing. point is raised with an increase in amount of previous cold reduction. For 92% reduction, with certain brands of electrolytic copper, the semi-annealing point occurs at about 250 C., and the copper has a Rockwell- B hardness then of approximately 76. The permissible Variation in time here is from ten minutes to three hours, which is ample to permit the semiannealing to be completed in commercial practice and to produce a uniformly tempered sheet.

The annealing temperature may also be varied within a limited range with corresponding changes in the time, without disturbing the final result. For example, copper sheets having a sufficient previous cold reduction, held at 250 C. for one hour would be satisfactory for cornice work, or the same copper held at 275 C. for fifteen minutes would give almost identical hardness effects. At 225 C. it would probably take ten hours to reach the same point. At 300 C. perhaps one minute would sufiice, although one hour would soften the material only slightly below the cornice temper range.

These figures are merely illustrative, for certain copper will have its semiannealing point around 200 C. and other copper, such as arsenical, or copper containing appreciable amounts of silver, may require 300 to 350 C. for the same results, but for any given lot of copper which has been subjected to the same predetermined cold reduction there is a combination of time and temperature at which the annealing can be stopped to produce a commercial temper Within the range of requirements for subsequent use.

Copper exposed to a temperature of 200 to 300 C. is discolored only and does not lose its surface texture. It will need only a light acid cleaning to produce a highly lustrous sheet of cold rolled texture, hardness, tensile strength and ductility. If the annealing is carried on in a controlled neutral atmosphere, the acid treatment of the surface of the sheet may even be omitted. At any rate, at the low temperature of this annealing only a small amount of oxide, usually only an iridescent discoloration, will be formed, which may be readily removed by suitable treatment.

In accordance with the present invention it is possible with copper which has been reduced more than 75% of its cross sectional area by cold working, by carefully choosing a temperature and a time of annealing, suited to the composition of the copper and its impurities, and its amount of cold reduction, to produce commercially uniform sheets which are the equivalent for all commercial purposes, to the so-called cold rolled cornice temper copper. This temperature will be well below the usual annealing point for the copper and below the heavy scaling point. Obviously, other hardness ranges may be obtained by varying the percentage of cold reduction and the time and temperature of annealing. The above is merely illustrative.

A suitable method of cold rolling a sheet of copper to produce a reduction in cross sectional area in excess of 50% and up to 98% is set forth in the copending application of Lawrence R. Clapp and George Walter James, entitledProcess of rolling metals, Serial No. 3 19,5390, filed March 25, 1929. This process comprises heavy reductions on small crowned rolls, controlling the heat generated by the work with suitable lubricating and cooling means, so that a substantially flat rolling surface is maintained, and varying the slippage between the work and the rolls through the application of suitable lubricants for the various passes, so that the horizontal component of the force exerted upon the sheet can is maintained sufiiciently low to prevent the sheet from being torn apart and the metal from pulling away from the point of rolling. Since the particular manner in which the cold reduction is effected forms no part (if the present invention, it Will not be described herein in detail.

The treatment of copper has been described by way of example only. The process may also be applied to other metals by making appropriate changes in the numerical values of the different steps.

It is to be noted that the production of a metal product having a required hardness above that of completely annealed metal is simplified in accordance with the present process since the annealing operation may be so controlled that the required hardness is produced directly. This eliminates the necessity for completely softening the material in the annealing operation with its attendant scaling and subsequently treating it again, as, for example, by a slight cold reduction, to raise the hardness to the desired poin Although certain novel features of the invention have been shown and described and are pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes in the several steps of the process and in its operation and in the form and details 0? the apparatus illustrated may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

l. The process of producing a copper sheet having a cornice temper which comprises cold working said sheet to effect a reduction in cross sectional area in excess of thereby bringing the hardness at semi-annealing point within the cornice temper range and heating said sheet to a temperature of 250 to 350 C., said temperature corresponding to the semi-annealing point and causing the sheet to become annealed to that point.

2. The process of producing sheet copper having a hardness between 68 and '74 on the Rockwell-B scale which comprises cold rolling said sheet to efiect a reduction in thickness in excess of 75% and annealing said sheet at a temperature of approximately 275 C. for a period of onequarter to three hours.

3. The process of treating copper to produce a metal product having a predetermined hardness between 62 and '76 on the Rockwell-l3 scale which comprises effecting cold reduction in cross sectional area in excess of 15% and then annealing said copper at a temperature of 200 C. to 350 C. to anneal the copper to the desired hardness value and stopping the annealing operation at that point.

e. In the treatment or" copper, the method of producing a copper sheet having a hardness value in excess of 62 on the Rockwell-B scale and which when subjected to an annealing temperature in excess of 350 C. will exhibit only relatively slight, gradual and uniform decreases in hardness, which consists in subjecting copper to cold reduction in excess of 75% of its cross sectional area and annealing same at a temperature of 150 to 350 C.

5. The process of forming a copper sheet product having a predetermined hardness value in excess of 62 on the Rockwell-B scale and which will sufier only slight, gradual, uniform decreases in hardness valu when subjected to annealing temperatures in excess of 350 C. which comprises severely cold working said copper sufiioiently to impart thereto a hardness value on the Rockwell-B scale approximately 50% in excess of said 

